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Injection study of a diesel engine fueled with pilot-ignited, directly-injected natural gas Larson, Chadwick R.

Abstract

The method of fueling diesel engines with high-pressure direct injection (HPDI) of natural gas reduces regulated emissions while maintaining diesel-cycle efficiency. The relative injection timing between the pilot and natural gas, the absolute injection timing, and the injection pressure were varied in operation of a modified heavy-duty single-cylinder diesel engine. The emissions studied included nitrogen oxides (NO[sub x]), carbon monoxide (CO), particulate matter (PM) and total hydrocarbons (THC). The engine was operated at three speeds and three equivalence ratios. The effects of changing exhaust pressure and boost pressure were also examined. Exhaust back pressure significantly affects all emissions, but not gross efficiency. For consistent effect on emissions, an absolute back pressure of 150 kPa was selected for the standard test procedure. The relative injection timing (RIT) significantly affects most emissions, but does not appear to affect PM or specific fuel consumption. With respect to RIT, burn duration correlates well with emissions. All pollutant emissions are at a minimum when the RIT is set to 1.8 ms for the conditions tested. The timing of the 50% cumulative heat release (HR50) is a good variable for relating NO[sub x] and efficiency to injection timing. The reduction of specific NO[sub x] emissions as a function of HR50 with retarded injection is approximately independent of equivalence ratio. Specific fuel consumption as a function HR50 is nearly independent of load and speed; the best fuel consumption occurs when HR50 is approximately 5° after top dead center. The effects of retarding absolute timing on CO depend upon the equivalence ratio. Total hydrocarbon emissions increase with excessively retarded absolute timing. The effects of timing on PM depend on speed and equivalence ratio. The relationship between NO[sub x] and efficency and HR50 does not change with injection pressure. Increasing injection pressure appears to increase mixing, which generally improves CO emissions at high equivalence ratio, but worsens THC and CO emissions at a low equivalence ratio, late timing. Moderately changing engine air flow-rates with a constant equivalence ratio does not significantly affect emissions.

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